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3.1 Lights

 
Physically based rendering claims to simulate reality as close as possible. In the real world every light emitter has a size and shape. Traditional light models in computer graphic like directional, point or spot light doesn't have this restriction and will be represented by icons. Light is coming out of one point in space.
 
The idea in Iray Photoreal is to use the image based lighting approach by using the nodes ifmImageBasedLighting or ifmHosekSkylight or modeling the shape of the light emitter and combine it with one of the light emissive materials available. To get a proper result when using physically based rendering the correct modelling of light emitting shapes is necessary. The closer the model fits to the real light the better the simulation. But in many situations this approach isn't required to get photorealistic and more important, great looking results. So it is legitimate to decrease the complexity of light emitting geometry as long as the result fits to the aimed look in the final rendering.
 
To get an idea how to simulate traditional light models in Iray Photoreal the following set up can be used. This gives an idea how to use the given tools to get the expected results while lighting a scene.
 

Direction light

Use an ifmHosekSkylight and turn the attribute Sky off. Set the attribute Azimuth to 90 and the attribute Saturation to 0. The attribute Ground Albedo should be set to black (R=0, B=0, G=0). Rotating the ifmHosekSkylight gives a similar result as using a Directional Light in Maya.
 

Point light

Create a polygon sphere, scale it down and assign an ifmOmniEmitter to it. Keep in mind that you might need to adjust the attribute Exponential Intensity to see any results.
 

Spot light

Create a planar polygon, scale it down and assign a ifmSpotEmitter to it. Keep in mind that you might need to adjust the attribute Exponential Intensity to see any results.
 

Light linking and shadows

Because in the real world every light creates a shadow and every objects casts and receives a shadow, light linking or object-specific shadow calculation in Iray is not possible.
 
Note: Since Iray for Maya 1.2 the standard Maya lights are supported.
 

Light linking and shadows

Because in the real world every light creates a shadow and every objects casts and receives a shadow, light linking or object-specific shadow calculation in Iray is not possible.